Use of wind turbines for generating electrical power is currently increasing while recent technical advances along with economics and legislation have made the use of wind turbines a viable generation alternative. Wind turbines range from small sizes of 1–50 kW for power generation for homes, farms and small business up to 5 MW in large scale off shore wind generating plants.
Wind turbines can utilize numerous constructions but are generally defined as either fixed speed or variable speed. Fixed speed wind turbines rotate at a fixed speed and employ an asynchronous or induction generator to produce power. The rotational rate of the turbine rotor is increased through the use of a speed increaser gearbox so that the induction generator produces either 50 or 60 Hz power. Fixed speed wind turbines allow for easy power connection because the generator can be directly connected to utility power grid.
Unfortunately, fixed speed wind turbines have numerous deficiencies. Operation at fixed speed puts increased stress and wear and tear on the turbine. The speed increaser gearbox reduces efficiency. The speed increaser gearbox is also the largest failure component of wind turbine operation and requires regular maintenance. Fixed speed operation is also very noisy especially in low wind when it is most noticeable. Although easily connected with utility power, the asynchronous generator provides low quality power with a low power factor. Induction generators also suffer from low efficiency and they do not generate power unless connected to grid power, eliminating the possibility of stand-alone generation.
Increasingly, use of variable speed wind turbines is becoming more preferred. Variable speed wind turbines rotate at speeds that vary with the wind speed. They can be designed to efficiently produce power even in reduced wind. As a result, variable speed wind turbines can capture more wind energy. In its most basic form, the turbine rotor drives a speed increaser gearbox that rotates a synchronous generator to produce power. The gearbox increases the rotational speed so that the generator can produce sufficient power. The output of the synchronous generator varies in voltage level and in frequency with the velocity of the wind. The power is rectified with a rectifier and then inverted with an inverter to provide high quality AC power.
To eliminate the reliability, maintenance and efficiency issues of the speed increaser gearbox in variable speed wind turbines, more recently, large diameter synchronous ring generators have been developed. Synchronous ring generators utilize a large diameter construction, in many cases over 3 meters, and have a very large number of magnetic poles. This construction allows them to generate high power even at low rotational speeds. The turbine rotor can then be directly coupled to drive the synchronous ring generator, without the use of a gearbox.
In general, wind turbines still need to overcome significant issues in order to reach their full potential for electrical power generation. The biggest challenge is the reduction of the cost per unit power generation capability. New wind turbines also need to reduce life cycle costs, particularly by increasing reliability and reducing required maintenance. Both are very significant concerns for enabling large wind farms and remote or off shore installations. Increased generation efficiency would further allow for production of more energy from wind turbines and would reduce the energy generation costs. Other areas for improvement also include noise reduction, especially for wind turbines located near residential areas.
Despite the development of variable speed wind turbines employing large synchronous ring generators, wind turbines continue to suffer from deficiencies. Most importantly, wind turbines with synchronous ring generators are very costly. This is due to both the use of a large amount of costly permanent magnets and/or electrical laminations and also to the very labor-intensive fabrication and assembly processes. Processes such as punching stacking, grinding, winding, insulating, coil inserting, coil connection and high tolerance alignment increase costs. On top of this, costly close tolerance components are required to achieve the required small magnetic airgaps for achieving efficient operation.
Other problems with current synchronous ring wind turbine generators include the need for extremely large and rigid generator structure and also assembly tooling to deal with the tremendous magnetic attraction forces between the rotor and stator portions. As a result of the massive structure, generators therefore become excessively heavy, making them more difficult to transport and to install. Wound field synchronous ring generators can eliminate the problem of rotor to stator attraction during assembly, but not in operation when field current is applied to the field coils for power generation. In addition, wound field generators are both more costly and achieve lower performance. Currently available synchronous ring generators suffer from less than optimal efficiency and provide limited power generation capability in reduced wind speeds. In many cases, substantial cooling systems are also required to cool the generator and remove energy that is wasted as heat. A new wind turbine with improved generator construction is needed.